Russia’s nuclear energy agency has said an explosion that caused radiation levels to spike in the Arkhangelsk region was caused by an accident during a test of an “isotope power source for a liquid-fuelled rocket engine”, The Guardian reports.

In a statement released late on Friday, August 9, Rosatom said five of its employees had died as a result of the accident and three more were being treated for burns.

The statement was the first confirmation that the agency was involved in the incident, which briefly drove radiation levels up to 20 times their normal levels in the nearby city of Severodvinsk.

Rosatom’s description of the incident could indicate it was testing the nuclear-powered cruise missile Burevestnik mentioned during a speech by Vladimir Putin last year.

Russia’s ministry of defence first confirmed the explosion on Thursday, saying two people had been killed and six injured in a botched test of a liquid-fueled rocket engine. The injured included ministry employees and civilian contractors.

Rosatom’s statement may indicate that three of those first reported as injured had since died. Despite reports that the wounded had been transported to Moscow for medical care, neither their names nor locations had been confirmed.

The explosion caused radiation levels to rise for about half an hour in Severodvinsk, contradicting official ministry of defence claims that there had been no rise in radiation levels. Severodvinsk removed a statement confirming the spike in radiation levels from its website, saying the ministry was managing the response to the incident.

While local officials urged calm, residents in several cities stocked up on iodine, which is often used to limit the effects of radiation exposure.

Russia also closed a bay in the White Sea to civilian traffic for a month, driving speculation that either the water had been contaminated or that a search operation was being launched.